1. Field of the Invention
The present invention relates to a wire bonding method in which a first bonding point and a second bonding point are connected by a wire, and more particularly to a wire loop formation method in wire bonding.
2. Prior Art
As seen from FIGS. 4(a) and 4(b), in a semiconductor device assembly process, a pad 2a (first bonding point) on a semiconductor chip 2 mounted on a lead frame 1 and a lead 1a (second bonding point) on the lead frame 1 are connected by a wire 3. FIG. 4(a) shows a connected wire shaped in a trapezoidal loop, and FIG. 4(b) shows a connected wire shaped in a triangular loop. Wire loop formation methods of these types are disclosed, for example, in Japanese Patent Application Publication (Kokoku) No.H5-60657 and Japanese Patent Application Laid-Open (Kokai) No. H4-318943.
The trapezoidal loop shown in FIG. 4(a) is formed by the process shown in FIG. 5.
In step (a), with a damper (not shown) which holds the wire 3 opened, the capillary 4 is lowered, so that a ball formed on the tip end of the wire is bonded to the first bonding point A, after which the capillary 4 is raised in substantially vertical direction to point B, and the wire 3 is delivered. Next, in step (b), the capillary 4 is caused to move horizontally to point C in the opposite direction from the second bonding point G. Generally, such movement of the capillary 4 in the opposite direction from the second bonding point G is called a xe2x80x9creverse operationxe2x80x9d. As a result of the reverse operation, the wire 3 assumes a shape that is inclined from point A to point C, and a kink 3a is formed in a portion of the wire 3. The wire 3 delivered in this process from point A to point C forms the neck height part 31 shown in FIG. 4(a).
Thereafter, in step (c) in FIG. 5, the capillary 4 is raised in substantially vertical direction to point D, and the wire 3 is delivered. Afterward, in step (d), the capillary 4 is again caused to move horizontally to point E in the opposite direction from the second bonding point G. In other words, a second reverse operation is performed. As a result, the wire 3 assumes a shape that is inclined from point C to point E, and another kink 3b is formed in a portion of the wire 3. This wire 3 delivered from point C to point E forms the trapezoidal length part 32 shown in FIG. 4(a).
Next, in step (e) of FIG. 5, the capillary 4 is raised in substantially vertical direction to point F, and an amount of wire 3 corresponding to the inclined portion 33 shown in FIG. 4(a) is delivered. Afterward, the damper (not shown) is closed. When the damper is closed, no wire 3 is delivered even if the capillary 4 subsequently is moved. Next, in steps (f) and (g), the capillary 4 is positioned at the second bonding point G by being caused to perform a circular-arc motion or by being lowered after being caused to perform a circular-arc motion, and the wire 3 is bonded to the second bonding point G.
On the other hand, the triangular loop shown in FIG. 4(b) is formed by the process shown in FIG. 6. Since this triangular loop is a loop in which the trapezoidal length portion 32 of the trapezoidal loop is not formed, the second reverse operation taken in step (d) of FIG. 5 is not performed. Accordingly, the steps (c), (d) and (e) in FIG. 5 are reduced to only the step (c) in FIG. 6. In other words, the steps (a) and (b) in FIG. 6 are the same as the steps (a) and (b) of FIG. 5; and after the first reverse operation in step (b) in FIG. 6, the capillary 4 is raised in step (c) in substantially vertical direction to point F and the wire 3 is delivered. Afterward, the capillary 4 performs the operations of steps (d) and (e), which are similar to the operations in steps (f) and (g) of FIG. 5, and the wire 3 is bonded to the second bonding point G.
As described above, the triangular loop formation process shown in FIG. 6 has advantages. The loop formation can be accomplished by a simpler process than the trapezoidal loop formation process shown in FIG. 5, so that loop formation can be accomplished in a shorter time.
However, in cases where the (vertical) height difference between the first bonding point A and the second bonding point G is large, or in cases where the first bonding point A and the end portion of the semiconductor chip 2 are (horizontally) widely separated, the wire 3 tends to contact the semiconductor chip 2 if the wire has a triangular wire loop shape shown in FIG. 4(b). In such cases, the trapezoidal wire loop shape shown in FIG. 4(a) is used so that contact between the wire 3 and the semiconductor chip 2 is prevented.
In the trapezoidal loop formation process shown in FIG. 5, the first reverse operation in step (b) is performed with the capillary 4 in a position that is close to the height of the first bonding point A. Accordingly, a comparatively strong kink 3a can easily be formed. However, the second reverse operation in step (d) is performed with the capillary 4 in a high position away from the first bonding point A. Accordingly, the kink 3b is difficult to form and is unstable. As a result, the area of the kink 3b shown in FIG. 4(a) is unstable, the shape retention strength of the wire loop is weak, and the height of the kink 3b is not aligned with the height of the kink 3a, forming an end-raised or end-lowered loop. Furthermore, if the shape retention strength of the portion in the vicinity of the kink 3b is weak, the bonded wire bends when pressure from the outside is applied to the wire. For example, wire bending tends to occur as a result of external forces such as shocks caused by capillary contact or ultrasonic oscillation during bonding to the second bonding point G, vibration of the wire 3, or mold flow caused by injection of the molding material during molding.
Accordingly, the object of the present invention is to solve the problem of trapezoidal loops, and the main object of the present invention is to provide a wire bonding method which forms a stable wire loop shape and a wire loop shape which has a high shape retention strength.
The above objects are accomplished by unique steps of the present invention taken for a wire bonding method in which a first bonding point and a second bonding point are connected by a wire that passes through a capillary; and in the present invention is involved: a step in which the tip end of a wire is connected to the first bonding point, a step in which a capillary is raised to deliver the wire and a first reverse operation is performed so that the capillary is moved in the direction opposite from the second bonding point; a step in which the capillary is raised to deliver the wire, caused to move toward the second bonding point and then lowered slightly; a step in which the capillary is raised to deliver the wire and a second reverse operation is performed so that the capillary is moved in the opposite direction from the second bonding point; and a step in which the capillary is raised to deliver the wire, after which the capillary is moved toward the second bonding point and the wire is connected to the second bonding point.
The above described method may include steps to slightly lower the capillary after the reverse operations.